1. Field of the Invention
The invention relates to a color cathode ray tube including an in-line type electron gun, and in particular, a color cathode ray tube which is capable of readily compensating for misconvergence.
2. Description of the Related Art
First, a structure of a conventional color cathode ray tube is explained below with reference to FIG. 1. The illustrated color cathode ray tube includes a bulb 1 comprised of a panel 1b, a neck portion 1c, and a funnel 1a, which has a truncated-conical cross-section and connects the panel 1b and the neck portion with each other.
A fluorescent filter film 2 is applied to an inner surface of the panel 1b. The flourescent film 2 includes flourescent materials for emission of three primary colors, which are separated from one another with photo-absorbing material sandwiched between them.
An in-line type electron gun 3 is installed in the neck portion 1c for emitting three electron beams to the flourescent film 2 to cause emission of three primary colors.
A deflecting yoke 4 is secured to the bulb 1 over the funnel portion 1a and the neck portion 1c. The deflecting yoke 4 is comprised of a bobbin 5 having a truncated-conical cross-section, a first coil 6 wound around the bobbin 5 for horizontally deflecting a magnetic field, a second coil 7 wound around the bobbin 5 for vertically deflecting a magnetic field, and a ferrite core 8 applied on an outer surface of the bobbin 5.
Though not illustrated, the color cathode ray tube further includes an inner shield in the funnel portion 1a, a shadow mask facing the flourescent film in the funnel portion 1a, and an aid such as a purity magnet in the neck portion 1c.
During operation, the electron gun 3 horizontally emits and accelerates three parallel electron beams, and deflects two other electron beams, between which a central electron beam is situated in such a manner that those two electron beams converge to the central electron beam.
Sawtooth current is supplied to each of the first and second coils 6 and 7, generating horizontally and vertically deflected magnetic fields.
The three electron beams emitted from the electron gun 3 enter the deflected magnetic fields, and are deflected to a degree proportional to the intensity of the magnetic fields. These three deflected electron beams are converged onto the flourescent film 2 to emit lights. As a result, colored images appear on the panel 1b.
In order to produce colored images having no color-misregistration, the three electron beams must be correctly directed to associated color regions in the fluorescent film 2.
However, it is quite difficult to make the deflected magnetic fields completely symmetrical in the bulb 1 because of a dispersion in the shape in the windings of the first and second coils 6 and 7, a dispersion in location of the first and second coils 6 and 7 when secured to the bobbin 5, a dispersion in the axis of the electron gun in the neck portion 1c, and/or a gap between axes of the deflecting yoke 4 and the electron gun 3. Accordingly, it is impossible to focus the three electron beams onto the fluorescent film 2, and the resulting misconvergence among the electron beams in turn results in misregistration of color on the fluorescent film 2.
This color misregistration considerably degrades the quality of images in a computer display. In order to prevent images from being degraded, the deflecting yoke 4 is set around the bulb 1 in a conventional cathode ray tube. A test pattern is displayed on the fluorescent film 2, and deflected magnetic fields generated by the deflecting yoke 4 are compensated for, so that the test pattern is displayed in a desired shape and in a desired color, and the generated images have no color misregistration.
Many attempts have been made to compensate for deflected magnetic fields. For instance, Japanese Unexamined Patent Publication No. 55-157846 suggests the deflecting yoke illustrated in FIG. 2. In the illustrated deflecting yoke, four magnetic pieces 9 are secured onto an outer surface of a bobbin 5. The magnetic pieces 9 are composed of iron alloy containing nickel as a principle ingredient (commercially available in the tradename of "PERMALLOY") and are equally spaced around the circumference of the bobbin 5. The magnetic pieces 9 improve coma-aberration on a screen, and compensate for color misregistration horizontally and vertically in three primary colors, red (R), blue (B), and green (G), as illustrated in FIG. 3.
Japanese Unexamined Patent Publication No. 8-115686 suggests a deflecting yoke for misconvergence. In the suggested deflecting yoke, illustrated in FIG. 4 , magnetic pieces 10 composed of magnetic material having high magnetic permeability, such as silicon steel and "PERMALLOY," are attached to an outer surface of the bobbin 5 in such a manner that the magnetic pieces are movable around the circumference of the bobbin 5.
Japanese Unexamined Patent Publication No. 9-45261 suggests a deflecting yoke as illustrated in FIG. 5. The illustrated yoke is formed with four slide rails 12 diagonally positioned reletive to the bobbin 5 at a rear end of the yoke. A magnetic piece 11 is supported along the slide rail 12. The magnetic pieces 11 are composed of silicon steel containing 3% silicon, or magnetic materials such as ferrite and amorphous providing the same effects as those of silicon steel. A part of the magnetic flux leaking out of the deflecting yoke is cut off by appropriately adjusting the magnetic pieces 11. As a result, a profile of magnetic flux density in the bulb 1 is adjusted, improving deformation of images.
As explained above, the conventional deflecting yokes can improve image deformation and/or color-misregistration that result from misconvergence.
If, as illustrated in FIG. 6A, a horizontally deflected magnetic field is asymmetrically distributed in the bulb 1 due to a dispersion in the shape in the windings of the first and second coils 6 and 7, a dispersion in location of the first and second coils 6 and 7 when secured to the bobbin 5, a gap between axes of the electron gun 3 and neck portion 1c, and/or a gap between axes of the deflecting yoke 4 and the electron gun 3, then a magnetic flux density in a horizontal direction also becomes asymmetrical, as illustrated in FIG. 6B with a solid line X1. Forces exerting on the electron beams R, G, and B also become asymmetric as a result.
Hence, an electron beam located at a distance S from the center at the right side receives a force from a magnetic field, a force which differs in magnitude from a force received by another electron beam located at the same distance from the center at the left side, and misconvergence is generated on the fluorescent film 2 between a central electron beam G and the other two electron beams 13B and 13R, as illustrated in FIG. 2C. In order to eliminate this misconvergence, it is necessary to adjust a profile of deflected magnetic flux in such a manner that a green bright line 13G, which is a reference line, is made closer to a bright blue line 13B or a red bright line 13R located inside or outside the green bright line 13G.
A horizontally deflected magnetic field is partially leaked outside the deflecting yoke 4. Therefore, if a magnetic piece having high magnetic permeability is positioned as a compensator in a leaked magnetic field at a rear of the deflecting yoke 4, the leaked magnetic flux is partially cut off, compensating for a profile of the magnetic flux in the bulb 1.
By moving the magnetic piece as a compensator, a profile of the magnetic flux density is differentially varied horizontally around the center when viewed from the panel 1b. As a result, the electron beams located at the distance S from the center receive the same magnitude force, which ensures elimination of misconvergence.
If a horizontally deflected magnetic flux is distributed asymmetrically out of a curve of the second order, as illustrated in FIG. 7A, misconvergence is generated between two electron beams sandwiching a central electron beam therebetween, as illustrated in FIG. 7B.
As illustrated in FIG. 7B, the blue and red electron beams, which sandwich a central electron beam, project two rectangles on the screen. A side of a rectangle overlaps a side of another rectangle, and the two rectangles cooperate with each other to put the red and blue bright lines 13R and 13B in optimal condition. However, vertical bright lines 13R and 13B located at a center of the screen are out of position.
In such a condition, even if a magnetic piece having high permeability is put in a leaked magnetic field and moved therein to vary a profile of a magnetic flux density in a bulb, it would be impossible to adjust a profile of magnetic flux density to be horizontally symmetric on a screen. If bright lines located at the center of the screen are overlapped, bright lines 13R and 13B, located at opposite sides, are offset with each other. As a result, any such adjustment ends up with the bright lines located at the opposite sides being incorrectly balanced.
In light of these problems, miconvergence can only be completely eliminated by means of other compensators (not illustrated) secured to the bulb, which brings about more problems with complicated adjustments and an increase in the number of compensation steps.